The present invention relates to a DC-DC converter, and more particularly, to a controller for a DC-DC converter and a method for controlling the DC-DC converter.
Portable electronic devices are recently being widely used. The portable devices use a battery for a drive power supply. Since the output voltage of the battery decreases through use of the device or through discharge, the electronic device includes a direct current voltage converting circuit (DC-DC converter) for converting the voltage of the battery to a constant voltage. The conversion efficiency of the DC-DC converter affects the operation time of the electronic device, which is operated by the battery. Since the electronic device requires different amounts of current depending on the operation at a given time, there is a demand for a DC-DC converter and controller for a DC-DC converter having satisfactory conversion efficiency irrespective of the change in current amount.
In the prior art, a portable electronic device uses a switching type DC-DC converter (hereinafter referred to as a switching regulator) that is compact and has satisfactory conversion efficiency. The switching regulator is a pulse width modulation (PWM) type regulator that keeps the output voltage substantially constant by controlling the pulse width of a pulse signal, which drives a main switching transistor according to the output voltage or the output current.
An electronic device operated by a battery may at times suppress the consumption current to prolong the life of the battery and extend the operation time of the electronic device. However, in such a low load state, the voltage conversion efficiency is extremely low in a PWM type switching regulator. This is due to the following reasons.
The power loss of the switching regulator includes loss that depends on the output current of the switching regulator and a fixed loss that does not depend on the output current of the switching regulator. The loss that depends on the output current includes power loss resulting from the ON-resistance of a transistor. The fixed loss that does not depend on the output current includes power loss resulting from the activation and inactivation of a main switching transistor and loss caused by power consumption of the switching regulator itself. When the output current is large (difference between the input current and the output current being small), the loss that depends on the output current becomes large. However, the conversion efficiency of the entire switching regulator is satisfactory since the fixed loss is constant. On the other hand, if the output current decreases (difference between the input current and the output current becoming large), the conversion efficiency decreases since the percentage of the fixed loss with respect to the loss depending on the output current increases.
The fixed loss of the switching regulator is proportional to the switching frequency for activating and inactivating the main switching transistor. Thus, the power loss may be reduced by lowering the switching frequency. However, this would increase ripple of the output voltage.
Japanese Laid-Open Patent Publication No. 2003-9515 describes a power supply system including a switching regulator and a linear regulator. The system switches between the two regulators in accordance with the load current. The loss of the linear regulator is determined by the difference between the input voltage and the output voltage and is not dependent on the difference between the input current and the output current. Thus, the linear regulator realizes high conversion efficiency in low load states.